1. Field of the Invention
The invention relates to protective-colloid-stabilized polymers in the form of their aqueous dispersions or in the form of their water-redispersible powders, to a process for preparing these polymers by emulsion polymerization, and to the use of these polymers.
2. Background Art
Protective-colloid-stabilized polymers are used especially in the form of their aqueous dispersions or of their water-redispersible polymer powders in a wide variety of applications, for example as coating compositions or in adhesives for a corresponding variety of substrates, e.g. as cementitious tile adhesives. The protective colloids generally comprise polyvinyl alcohols. The use of polyvinyl alcohol is desired because the polyvinyl alcohol itself contributes to strength, for example to tensile bond strength in tile adhesives, whereas this is not the case in systems stabilized by low-molecular-weight compounds (emulsifiers). Monomers preferably used hitherto for the preparation of redispersible powders are vinyl esters and ethylene, due to the fact that stabilization of acrylate copolymers, styrene-acrylate copolymers, and styrene butadiene copolymers is much more difficult to achieve by polyvinyl alcohols.
One disadvantage of the use of polyvinyl alcohol is that this protective colloid gives the resultant products low water resistance. Problems can arise not only during preparation but also during the use of the desired products. In particular, when polymers in the form of their redispersion powders are used to improve mortar properties, a primary application sector of redispersion powders, the formulations have to remain stable over a certain time, and must suffer no significant alteration in their usage consistency (viscosity stability or cement stability), because the user should not expect to have to make another mix after only a short period. In the concrete and mortar industry, furthermore, mechanical properties such as compressive strength and porosity and the associated air-pore content play an important part. If too many air pores are present, compressive strength falls markedly, while if too few, or no, air pores are present in the mortar or concrete, the construction material lacks adequate resistance to freeze-and-thaw cycles. In addition, the hydraulically setting systems whose quality has been improved by the dispersion powder are intended to provide even better adhesion than systems whose quality has not been thus improved.
Although polyvinyl alcohol-stabilized powders have good viscosity properties and processing properties in cementitious applications, further improvement of this performance is desirable in many cases. Polyvinyl-alcohol-stabilized powders and dispersions are widely mentioned in the literature, for example in EP-A 1065224, EP-A 1110978, and EP-A 1110979. In order, first to increase the water resistance of the resultant products, and second, to increase the stability of the dispersions, WO-A 99/16794, for example, describes the use of polyvinyl alcohols hydrophobicized with VeoVa10®. Although these polyvinyl alcohols provide excellent stabilization, they have only very little effect on the water resistance of the resultant products, because, with respect to water-solubility, the degree of modification is very low.
Other attempts to improve the properties mentioned have been based, for example, on water-soluble polyvinyl acetals. These are obtained by reacting polyvinyl alcohol with aldehydes. A problem with these polymers is the fact that some of them have very low cloud points and therefore have only limited use, or cannot be used at all in the polymerization process. The hydrophobicizing properties of these polymers are good, but the stabilizing action is only moderate. Polyvinyl acetal protective colloids are described in EP-A 834520.
EP-A 1088835 describes the use of water-soluble, fully hydrolyzed ethylene-containing polyvinyl alcohols as protective colloids in the polymerization process. The fully hydrolyzed products described lead to binders with good adhesive properties for the adhesive-bonding of wood, in particular improved water resistance of the adhesive bond. However, these products are not suitable for preparing dispersion powders, because the water resistance has been increased to the extent that the powder no longer redisperses.